Studies of the Mechanisms of the Human Platelet Release Reaction Induced by Immunologic Stimuli

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Abstract
Human platelets in the presence of plasma show a biphasic response to immune complexes (IC) formed from either antigen and antibody or from aggregated immunoglobulin. Rapid release of serotonin within the 1st min is unaffected by cobra venom factor (CVF), EDTA, and agents which prevent the aggregating action of ADP, but is inhibited by complement (C) components. This release appears to be the direct interaction of platelets and IC analogous to release by washed platelets which is inhibited by C1. Aggregation follows, accompanied by a second release which requires ADP and is inhibited by CVF and EDTA. It is dependent on C, activated most probably via the classical pathway. Zymosan (Z) causes aggregation and release with the characteristics of the second phase of IC-induced release. This reaction, which occurs after an 8 to 10 minute lag phase, has an absolute requirement for C components which may be activated by the C3 shunt. Z binds the necessary components such that the resulting complex (Z-X) induces aggregation and release in a washed platelet system. Divalent metal ions and ADP are necessary for platelet response to Z-X.

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